Hurricane Irene has claimed its first fatality, battering the North Carolina coast with wind and rain and wreaking havoc as it began a potentially catastrophic run up the US east coast.
More than two million people were told to flee and the New York City transit system was shutting down for the first time because of a natural disaster.
The National Hurricane Centre in Miami said Irene's maximum sustained winds were around 85mph on Saturday morning, down from about 100mph a day earlier.
But they warned the hurricane would remain a large and powerful one as it trekked toward the mid-Atlantic coast over Saturday night and southern New England on Sunday.
Authorities said a man in North Carolina was killed outside his home by a tree limb blown down by the hurricane.
Tropical storm conditions have spread into coastal Virginia, Maryland, and Delaware. Hurricane-force winds first arrived near Jacksonville, North Carolina, at dawn.
As the storm's outer bands of wind and rain lashed the coast, knocking out power in places, authorities further north begged people to get out of harm's way.
"Don't wait. Don't delay," said President Barack Obama, who cut short his summer holiday and returned to Washington. "I cannot stress this highly enough: If you are in the projected path of this hurricane, you have to take precautions now."
Obama has declared a state of emergency for North Carolina, Virginia, New Jersey, New York, Connecticut and Massachusetts. Hurricane warnings were issued from North Carolina to New York and farther north to the islands of Nantucket and Martha's Vineyard off Massachusetts. Evacuation orders covered at least 2.3 million people.
Wind and rain knocked out power to more than 91,000 customers along the North Carolina coast, including a hospital in Morehead City which was running on generators. Forecasters said a tropical storm warning has also been issued for parts of Canada north of the US border as Hurricane Irene moves north.